Mid-infrared (mid-IR or MIR, λ=3-25 μm) light has been used as an imaging technique for a variety of applications, including medical diagnosis, night vision, and remote sensing. Mid-IR imaging provides some advantages over other IR, visible, and/or ultra-violet (UV) light-based imaging. Many substances absorb mid-IR light and cause resonant vibrational excitations of chemical bonds, which makes it a nearly universal label-free tool applicable to almost any material. Additionally, different compounds exhibit unique absorption features in the mid-IR spectrum, enabling mid-IR light to be used in spectroscopic applications. Mid-IR spectroscopy can be used to identify and characterize the chemical composition of objects. Additionally, mid-IR spectroscopy can be used to track activity of biological systems.
Mid-IR light also experiences significantly less scattering than visible or UV light, allowing it to penetrate much farther into scattering materials. Such reduced scattering can be useful when conducting measurements over a long distance, such as in gas sensing and remote sensing. Additionally, since any object that emits heat—at least above the temperature of absolute zero—radiates infrared light, mid-IR imaging can also be achieved without a separate illumination source. The object being imaged can itself serve as a light source and thus enable “passive-detection.” Further, since the photon energy of MIR radiation is relatively low, no photochemical reactions are stimulated, allowing mid-IR imaging to be non-disruptive. Other biological system imaging techniques, such as fluorescence imaging, suffer from photo-bleaching and phototoxicity effects.
Although mid-IR imaging can be a powerful tool, there are some drawbacks inherent to the physical nature of mid-IR light and mid-IR devices. One difficulty in mid-IR imaging is the scarcity of the infrared detectors. There are two major types of infrared detectors: thermal detectors and photonic detectors. Thermal detectors are typically affordable, but slow and sometimes inaccurate. Photonic detectors, on the other hand, have better read out speed and accuracy compared to thermal detectors, but are usually expensive and require an extremely low temperature to function (e.g. −203° C.). Commonly used indium antimonide (InSb) or mercury cadmium telluride (MCT) detectors usually requires a liquid nitrogen (LN2) or thermoelectric (TE) cooling system, which make them inconvenient to use and economically undesired.
Typical mid-infrared imaging techniques also tend to have limited spatial resolution, due to the Abbe diffraction limit. The spatial resolution of a specific color of light is linearly dependent on its wavelength, such that longer wavelengths lead to lower spatial resolution. Mid-IR light includes wavelengths from 3 to 25 μm, while visible light spans from 390 nm to 700 nm. This means that, in typical imaging systems, the spatial resolution achievable from mid-IR light can be anywhere from 5 to 65 times lower than that of visible light. This means that for biological systems, conventional MIR imaging typically provides access to spatial information down to the tissue level. One important application at this level known as “thermography” can be used, for example, as a diagnosis tool for early breast cancer detection. However, analysis of nano-scale structures is usually not possible with conventional MIR techniques. Due to these physical limitations, imaging submicron-sized objects using mid-IR light has been infeasible in existing imaging systems.
The apparent problem of low resolution in mid-IR imaging has been approached using different technologies, such as Scanning Probe Microscopy (SPM), solid-immersion lens, scattering-type scanning near-field optical microscopes (s-SNOM, a technique that utilizes a metalized atomic force microscope tip to scatter broadband infrared radiation), and others. However, those methods rely heavily on integration of sophisticated instruments and they deprive MIR measurement of its potential to be performed in media. At the same time, the cumbersome (cooling needed), expensive and less reliable MIR detectors are in the standard configuration of those methods.
Additionally, Fourier transform spectroscopy of the scattered light gives information about molecular vibrations with a spatial resolution as high as 20 nm. Another alternative form of SPM integration with MIR imaging relies on photothermal induced resonance effects. In this method, pulsed MIR light is used in total internal reflection with a ZnSe crystal. Absorption of the MIR light by the material in contact with the prism causes thermal mechanical expansion, which is detected by the SPM tip. Scanning the tip over the sample yields super-resolved MIR images. Also, in order to tackle the limited resolution, solid immersion lens is a viable approach too. MIR radiation is focused by the solid immersion lens into a material with a high refractive index, which thereby results in a reduced focal spot by a factor of the refractive index, comparing to that in vacuum. The evanescent wave escapes from the high refractive index material into the air, and hence probes the sample on the surface in a near field mode.
Although advances have been made with the methods described above, those techniques rely heavily upon the integration of sophisticated instrumentations, which results in complex experimental setups. Also, none of the approaches above overcomes the difficulty of detector sensitivity. A traditional expensive LN2- or TE-cooled detector is used within the standard configuration of those experimental setups. Thus, a method that not only improves the spatial resolution of MIR imaging, but also circumvents the usage of a standard MIR detector, is desired.
It is accordingly an objective of the present invention to provide mid-IR imaging systems and methods—which utilize the spectroscopic benefits of mid-IR—while also improving upon the spatial resolution limitations inherent in typical mid-IR imaging systems.